  Measurements on helium and argon gas flow through an array of parallel,
linear channels of 12 nm diameter and 200 micrometer length in a single
crystalline silicon membrane reveal a Knudsen diffusion type transport from
10^2 to 10^7 in Knudsen number Kn. The classic scaling prediction for the
transport diffusion coefficient on temperature and mass of diffusing
species,D_He ~ sqrt(T), is confirmed over a T range from 40 K to 300 K for He
and for the ratio of D_He/D_Ar ~ sqrt(m_Ar/m_He). Deviations of the channels
from a cylindrical form, resolved with transmission electron microscopy down to
subnanometer scales, quantitatively account for a reduced diffusivity as
compared to Knudsen diffusion in ideal tubular channels. The membrane
permeation experiments are described over 10 orders of magnitude in Kn,
encompassing the transition flow regime, by the unified flow model of Beskok
and Karniadakis.
