  Using the Kubo formula, we show explicitly that a non-interacting electron
system can not behave like a Hall-insulator, {\it ie.,} a DC resistivity matrix
$\rho_{xx}\rightarrow\infty$ and $\rho_{xy}=$finite in the zero temperature
limit, as has been observed recently in experiment. For a strongly interacting
electron system in a magnetic field, we illustrate, by constructing a specific
form of correlations between mobile and localized electrons, that the Hall
resistivity can approximately equal to its classical value. A Hall-insulator is
realized in this model when the density of mobile electrons becomes vanishingly
small. It is shown that in non-interacting electron systems, the
zero-temperature frequency-dependent conductacnce generally does not give the
DC conductance.
