Ballistic conduction or Ballistic transport is the transport of electrons in a medium with negligible electrical resistivity due to scattering. Without scattering, electrons simply obey Newton's second law of motion at non-relativistic speeds.
In general, the resistivity exists because an electron, while moving inside a medium, is scattered by impurities, defects, or by the atoms/molecules composing the medium that simply oscillate around their equilibrium position (in a solid), or generically by any freely moving atom/molecule composing the medium, in a gas or liquid.
For a given medium one can associate to a moving electron a mean free path as the average length that the electron can travel freely, i.e. before hitting against something and deviating from its original path, possibly losing some kinetic energy. The mean free path can be increased by reducing the number of impurities in a crystal or by lowering its temperature (except for some material like semi-conductors).
Ballistic transport is observed when the mean free path of the electron is (much) bigger than the size of the box that contains/delimits the medium through which the electron travels, such that the electron alters its motion only by hitting against the walls. In the case of a wire suspended in air/vacuum the surface of the wire plays the role of the box reflecting the electrons and preventing them from exiting toward the empty space/open air. This is because there is an energy to be paid to extract the electron from the medium (work function).
E.g. ballistic transport can be observed in a metal nanowire: this is simply because the wire is of the size of a nanometer ( meters) and the mean free path can be bigger than that in a metal
Ballistic conduction is the unimpeded flow of charge or energy carrying particles over relatively long distances in a material. Normally, transport of electrons (or holes) is dominated by scattering events, which relax the carrier momentum in an effort to bring the conducting material to equilibrium. Thus, ballistic transport in a material is determined by how ballistically conductive that material is. Ballistic conduction differs from superconductivity due to the absence of the Meissner effect in the material. A ballistic conductor would stop conducting if the driving force is turned off, whereas in a superconductor current would continue to flow after the driving supply is disconnected.
Ballistic conduction is typically observed in quasi-1D structures, such as carbon nanotubes or silicon nanowires, because of extreme size quantization effects in these materials. Ballistic conduction is not limited to electrons (or holes) but can also apply to phonons. It is theoretically possible for ballistic conduction to be extended to other quasi-particles, but this has not been experimentally verified.
Read more about Ballistic Conduction: Importance of Ballistic Conductivity, Optical Analogies of Ballistic Conduction, Examples of Ballistic Conduction