Incoherent interlayer conductivity of layered systems

The criterion for the maximum incoherent conductivity $\sigma_{c,max}$ in the anisotropic system is shown to be $\sigma_{c,max} \sigma_{ab}=C (e^2/h)^2 n_{2D}$, where $n_{2D}$ is the two-dimensional electron density, and $C$ is a numerical constant of order one. A model of c-axis interlayer conductivity of layered system in the incoherent regime is developed. The temperature dependence of the c-axis conductivity is derived to be $\sigma_c (T) \sim T^\alpha$, where $\alpha$ is determined by the effective strength of the interplane voltage fluctuations. The ``pseudogap'' phenomenon of suppressed interplane conductivity at low frequencies (or low voltages) is discussed.