  We investigate theoretically and experimentally the center-of-mass motion of
an ideal Fermi gas in a combined periodic and harmonic potential. We find a
crossover from a conducting to an insulating regime as the Fermi energy moves
from the first Bloch band into the bandgap of the lattice. The conducting
regime is characterized by an oscillation of the cloud about the potential
minimum, while in the insulating case the center of mass remains on one side of
the potential.
