  A semiclassical theory is developed for time-dependent current fluctuations
in mesoscopic conductors. The theory is based on the Boltzmann-Langevin
equation for a degenerate electron gas. The low-frequency shot-noise power is
related to classical transmission probabilities at the Fermi level. For a
disordered conductor with impurity scattering, it is shown how the shot noise
crosses over from zero in the ballistic regime to one-third of the Poisson
noise in the diffusive regime. In a conductor consisting of n tunnel barriers
in series, the shot noise approaches one-third of the Poisson noise as n goes
to infinity, independent of the transparency of the barriers. The analysis
confirms that phase coherence is not required for the occurrence of the
one-third suppression of the shot noise. The effects of electron heating and
inelastic scattering are calculated, by inserting charge-conserving electron
reservoirs between segments of the conductor.
