c-axis normal conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ as a function of x

A theory is presented of the static c-axis conductivity of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in the normal state with varying oxygen concentration. The basis of the theory is an assumption that the hopping of electrons between the ${\mathrm{CuO}}_2$ planes is due to resonant tunneling through localized centers between the planes. Comparison with recent systematic determinations of the temperature-dependent conductivity at different oxygen concentrations shows reasonable agreement.