  In this paper we show an approach to electron transport in double barrier
structures which unifies the well known sequential and resonant tunneling
models in the widest range of transport regimes, from completely coherent to
completely incoherent. In doing so, we make a clear distinction between
``approaches'' and ``transport regimes,'' in order to clarify some ambiguities
in the concept of sequential tunneling. Scattering processes in the well are
accounted for by means of an effective mean free path, which plays the role of
a relaxation length. Our approach is based on a recently derived formula for
the density of states in a quantum well, as a function of the round trip time
in the well and of trasmission and reflection probabilities for the whole
structure and for each barrier.
