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 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 (ML). (C) 2004 Elsevier B.V. All rights reserved.