Quantum kinetics in layered systems

Electron transport in strongly anisotropic materials exhibit several unusual properties.In particular, the resistivity across the layers (c-axis) has either an insulating-like or non-monotonic temperature dependences, whereas the resistivity along the layers is metallic. It is generally believed that when the scattering rate, 1/$\tau$, becomes larger than the tunneling rate between the layers, J, the nature of transport changes from coherent (band-like) to incoherent (tunneling-like). By using a Prange-Kadanoff--like quantum Boltzmann equation for electrons coupled to phonons, we show that there is no coherent/incoherent crossover for any value of J$\tau$, as long as the usual ``good metal'' condition is satisfied, i.e., E$_F\tau\gg$ 1. In other words, a strongly anisotropic metal is as ``coherent,'' as an isotropic one. The situation is changed in the presence of resonant tunneling centers between the layers. We show that the unusual behavior of the c-axis resistivity can be explained by inelastic resonant tunneling through such centers.