The density functional method is used to show that the description of molecular transport in subnanometer channels reduces to the description of diffusion in a one-dimensional system where strong density fluctuations with a finite cluster lifetime. A new diffusion mechanism is proposed; it makes it possible to explain the transition from activated diffusion of single particles in a channel at a low filling factor to fast barrier-free diffusion, which consists of the propagation of density disturbances at high filling factor. It is shown that as the filling factor increases, the attraction between the molecules (the effective attraction of molecules-hard spheres) causes the energy barrier for diffusion along the channel axis to vanish. Another consequence of the "effective" attraction between the molecules is the formation of molecular clusters in the channel, which possess a finite lifetime because of the one-dimensionality of the system. The size and lifetime of the clusters increase with the filling factor of the channel. The diffusion of particles in clusters is a barrier-free process of propagation of density disturbances. The dependences obtained for the diffusion coefficient on the pressure, temperature, and filling factor make it possible to describe, even in the hard-sphere model, all experimental data known to the authors.
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