  A formalism is proposed to study the electron tunneling between extended
states, based on the spin-boson Hamiltonian previously used in two-level
systems. It is applied to analyze the out--of--plane tunneling in layered
metals considering different models. By studying the effects of in--plane
interactions on the interlayer tunneling of electrons near the Fermi level, we
establish the relation between departure from Fermi liquid behavior driven by
electron correlations inside the layer and the out of plane coherence. Response
functions, directly comparable with experimental data are obtained.
