Analysis of anisotropic transport data of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ shows that the out-of-plane resistivity ${\mathrm{\rho }}_{\mathit{c}}$ changes from nonmetallic to metallic following a reduction in ${\mathrm{\rho }}_{\mathit{c}}$. The metallic ${\mathrm{\rho }}_{\mathit{c}}$ arises from a conventional relaxation mechanism within the framework of the band theory, while the nonmetallic ${\mathrm{\rho }}_{\mathit{c}}$ is dominated by hopping conduction. Whether the Mott-Ioffe-Regel rule is satisfied by the out-of-plane transport draws a distinction between relaxation transport and hopping conduction. The results can account for experimental data observed on single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ as well as other high-${\mathit{T}}_{\mathit{c}}$ copper oxides.

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