  The Kubo formula for the conductance of a mesoscopic system is analyzed
semiclassically, yielding simple expressions for both weak localization and
universal conductance fluctuations. In contrast to earlier work which dealt
with times shorter than $O(\log \hbar^{-1})$, here longer times are taken to
give the dominant contributions. For such long times, many distinct classical
orbits may obey essentially the same initial and final conditions on positions
and momenta, and the interference between pairs of such orbits is analyzed.
Application to a chain of $k$ classically ergodic scatterers connected in
series gives the following results: $-{1 \over 3} [ 1 - (k+1)^{-2} ]$ for the
weak localization correction to the zero--temperature dimensionless
conductance, and ${2 \over 15} [ 1 - (k+1)^{-4} ]$ for the variance of its
fluctuations. These results interpolate between the well known ones of random
scattering matrices for $k=1$, and those of the one--dimensional diffusive wire
for $k \rightarrow \infty$.
