= Molecular flow
In <molecular flow>, the <gas> <mean free path> is large compared with the channel or vessel dimensions, so <molecules> encounter walls more frequently than one another. The <Knudsen number> $\mathrm{Kn}=\lambda/L$ characterizes this regime.
For a small aperture of area $A$ between <equilibrium> <gas> reservoirs, the <vacuum conductance> is $C=A\overline v/4$, where $\overline v$ is the mean molecular speed. At fixed <temperature> this scales as the inverse <square root> of <molecular mass>. Continuum viscous-flow formulas need not apply.<Vacuum Technology for Ion Sources by Paolo Chiggiato>[molecular flow, aperture conductance]{ref}
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