= Solution
Ignoring access effects and wall charge, the cylindrical conducting liquid has
$$
\boxed{R=\frac{\ell}{\sigma\pi r^2}=\frac{100\times10^{-9}}{1.25\pi(5\times10^{-9})^2}=1.02\times10^9\,\Omega.}
$$
The corresponding conductance is approximately $0.982\,\mathrm{nS}$. A charged surface can contribute mobile <counterions> and change this estimate. Surface conduction is important when its conductance is comparable with the bulk contribution: for surface conductance $K_s$, their ratio in a cylinder is $2K_s/(\sigma r)$, twice the <Dukhin number> convention $K_s/(\sigma r)$. Double-layer overlap, when the <Debye length> is comparable with the pore radius, is another warning that uniform bulk conductivity is inadequate. No surface-charge or ionic-strength data are supplied to quantify those corrections.
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