When I became a sophomore about 38 years ago, I had for the first time heard the word "mean free path" of a gas molecule. Then when I became a junior next year, I had been taught about an elementary kinetic theory of gas. That kinetic theory was based on a classical model. That is, the gas was treated to be constituted from a collection of freely flying hard villiard balls. The concept of the "mean free path" means the average path length of a representative one villiard ball from one collision with other villiard ball to the next collision with the third villiard ball. In this picture, every gas molecule is considered to draw a definite deterministic flight path.
But in the recent experiments, especially in the double-slits experiments for atom or molecule, it was shown that the atom or the molecule behaves as a wave when it is flying freely. That is, all experiments showed the interference pattern that proves the quantum nature of the freely flying atom or molecule. As is often said, quantum entity travels as a wave, but arrives as a particle. So, except at the instance of collision, the freely flying gas molecule needs to be described as a propagating probability wave. Since a wave is an entity that is spreading over the space by its definition, it is difficult to draw or assume a definite straight flight path of the gas molecule.
Therefore, the kinetic theory of gas might be needed to be rewritten based on the quantum mechanical principles like the uncertainty principle, the concept of probability wave, the concept of the wave collapse, etc. It must become a quite new kinetic theory that even J. C. Maxwell and L. Boltzman could not expect.