= Access resistance of a circular pore
{title2=$R_{\rm access,one\ side}=\rho/(4a)$}
The continuum spreading resistance for current entering a circular aperture of radius $a$ from one uniform semi-infinite reservoir of <electrical resistivity> $\rho$ is $\rho/(4a)$. Two symmetric reservoirs contribute $\rho/(2a)$ in series with a cylindrical pore resistance $\rho\ell/(\pi a^2)$. Unobstructed access is not the same as zero spreading resistance; molecular-scale pores also require non-continuum transport physics.
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