  The metal-insulator transition (MIT) is an exceptional test bed for studying
strong electron correlations in two dimensions in the presence of disorder. In
the present study, it is found that in contrast to previous experiments on
lower-mobility samples, in ultra-high mobility SiGe/Si/SiGe quantum wells the
critical electron density, $n_{\text{c}}$, of the MIT becomes smaller than the
density, $n_{\text{m}}$, where the effective mass at the Fermi level tends to
diverge. Near the topological phase transition expected at $n_{\text{m}}$, the
metallic temperature dependence of the resistance should be strengthened, which
is consistent with the experimental observation of more than an order of
magnitude resistance drop with decreasing temperature below $\sim1$ K.
