Out-of-plane conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Pressure measurements on successively heat-treated ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals suggest that the out-of-plane resistivity (${\mathrm{\rho }}_{\mathit{c}}$) is diffusive even if d${\mathrm{\rho }}_{\mathit{c}}$/dT is positive in high-quality samples. d${\mathrm{\rho }}_{\mathit{c}}$/dT>0 comes from the temperature dependence of the in-plane lifetime of the particles which tunnel to the neighboring planes and d${\mathrm{\rho }}_{\mathit{c}}$/dT<0 is due to phonon-assisted hopping when disorder is introduced between the CuO layers.