  By tracking the delocalized states of the two-dimensional hole gas in a
p-type GaAs/AlGaAs heterostructure as a function of magnetic field, we mapped
out a phase diagram in the density-magnetic-field plane. We found that the
energy of the delocalized state from the lowest Landau level flattens out as
the magnetic field tends toward zero. This finding is different from that for
the two-dimensional electron system in an n-type GaAs/AlGaAs heterostructure
where delocalized states diverge in energy as B goes to zero indicating the
presence of only localized states below the Fermi energy. The possible
connection of this finding to the recently observed metal-insulator transition
at B = 0 in the two-dimensional hole gas systems is discussed.
