  Using several independent methods, we find that the metal-insulator
transition occurs in the strongly-interacting two-valley two-dimensional
electron system in ultra-high mobility SiGe/Si/SiGe quantum wells in zero
magnetic field. The transition survives in this system in parallel magnetic
fields strong enough to completely polarize the electrons' spins, thus making
the electron system "spinless". In both cases, the resistivity on the metallic
side near the transition increases with decreasing temperature, reaches a
maximum at a temperature $T_{\text{max}}$, and then decreases. The decrease
reaches more than an order of magnitude in zero magnetic field. The value of
$T_{\text{max}}$ in zero magnetic field is found to be close to the
renormalized Fermi temperature. However, rather than increasing along with the
Fermi temperature, the value $T_{\text{max}}$ decreases appreciably for
spinless electrons in spin-polarizing magnetic fields. The observed behavior of
$T_{\text{max}}$ cannot be described by existing theories. The results indicate
the spin-related origin of the effect. At the same time, the low-temperature
resistivity drop in both spin-unpolarized and spinless electron systems is
described quantitatively by the dynamical mean-field theory.
