  In the classical transport theory, the coefficients such as the diffusion,
thermal conductivity and viscosity of fluid are usually expressed in a form
proportional to the mean free path of molecules. We point out that this may
cause a great misunderstanding in the molecular transport theory and prove from
multiple perspectives of theory and simulation that the molecular transport
coefficient is actually proportional to the mean square free path, i.e., the
second moment of the free path distribution function. For two systems with the
same mean free path but different molecular free path distributions, the
classical expression gives the same transport coefficient, whereas the
expression with the mean square free path predicts different transport
coefficients, in which their difference may vary from zero to several times as
the distribution function becomes more dispersed.
