  The gate bias dependency of conductivity is examined in two Si quantum wells
with well thickness tw = 7 nm and tw = 14 nm. The conductivity of the thinner
device behaves smoothly whereas the thicker device shows strong non-monotonic
features as a function of gate voltages. We show that a strong minimum in
conductivity occurs close to the threshold of second sub-band. Another minimum
is seen at high electron density at symmetric well potential. This feature is
addressed to sub-band wave function delocalization in the quantization
direction.
