  We propose a model for the description of the transport properties of
metallic films on a large scale of slab thickness. This model is based on
solving the linearized Boltzmann equation in relaxation-time approximation
using {\it ab initio} calculations within the framework of the density
functional theory. The expression for the relaxation time is derived from the
microscopic treatment of the scattering processes and provides the correct
thickness dependence for very thin as well as very thick films. The method is
applied to the calculation of the in-plane conductivity and the Drude-type
plasma frequency of thin Cu(001) films in the thickness range between 1 and 32
monolayers.
