  In this work, we present a theoretical study of the statistical properties of
wave scattering in a disordered ballistic waveguide of length L; we have called
this system the "building block". The building block is interesting as a
physical system because its statistical properties could be studied
experimentally in the laboratory.
  In order to study the building block, as a physical system in itself, we have
developed a perturbative method based on Born series. This method is valid only
in the ballistic regime, when the length of the system L is smaller than the
mean free path l, and in the short-wave-length approximation, when the the wave
number k and the mean free path l satisfy kl >> 1. This method has allowed to
find, analytically, the behavior of quantities of interest that we have not
been able to find from the diffusion equation. In contrast with the diffusion
equation method, which takes into account approximately the contribution of
closed channels, this method takes them explicitly. In earlier works numerical
evidence was found that the expectation values of some interesting quantities
are insensitive to the number of closed channels that were used on the
calculations; with this method, we could show that closed channels are relevant
for the expectation values of amplitudes but irrelevant for the intensities and
conductance expectation values. The results of this method show a good
agreement with numerical simulations.
