  For a two dimensional electron system in a strong perpendicular magnetic
field B, there are extended states at each Landau level. We show that if the
inverse compressibility is negative, then the extended states float downward in
energy when B decreases. We set up a condition for the case where all extended
states are below the fermi energy in the low B limit. The condition may explain
why a conducting state has been observed in a high mobility Si-Mosfet but so
far not in high mobility n-GaAs.
