Studying the diffusion of gases within nanopores is an area of intense research due to the practical importance of diffusion in the areas of catalysis and separations, as well as the emerging interest in diffusion in carbon nanotubes. In the large Knudsen number limit, i.e., the rarefied gas state, which is the regime of interest, the self-diffusivity and a closely related collective property known as the transport diffusivity become equivalent. As such, the collisions between gas molecules are extremely rare and do not contribute to Ds,x.