We study the subband structure in a smooth film where isotropic volume scattering gives rise to a finite mean free path. The intersubband transitions into both the propagating and evanescent modes are described by an infinite system of equations. Its solution determines self-consistently the scattering-induced level broadening. The thickness dependence of the density of states and the conductivity are discussed in detail and compared with results which follow from the neglect of level broadening. For typical mean free paths, level broadening strongly smears out the steps of the density of states and suppresses size-induced oscillations of the conductivity. These results are in agreement with the general trend in experimental observations.