  The dependence of the localization length on the number of occupied subbands
$N$ in low-dimensional semiconductors is investigated. The localization length
is shown to be proportional to the number of occupied subbands in
quasi-one-dimensional quantum wires while it grows exponentially with $N$ in
quasi-two-dimensional systems. Also a weak localization theory is developed for
large N with a well-defined small expansion parameter $1/N$. The temperature
dependence of the conductivity deduced using this perturbation theory agrees
with the experimentally observed dependence.

Condensed Matter