  The mean free path is an essential characteristic length in disordered
systems. In microscopic calculations, it is usually approximated by the
classical value of the elastic mean free path. It corresponds to the Boltzmann
mean free path when only isotropic scattering is considered, but it is
different for anisotropic scattering. In this paper, we work out the
corrections to the so called Boltzmann mean free path due to multiple
scattering effects on finite size scatterers, in the s-wave approximation, ie.
when the elastic mean free path is equivalent to the Boltzmann mean free path.
The main result is the expression for the mean free path expanded in powers of
the perturbative parameter given by the scatterer density.
