  Two recent electronic transport experiments from Columbia University and
Harvard University have reported record high mobility and low channel densities
in transition metal dichalcogenide (TMD) WSe$_2$ monolayers [J. Pack, et al.,
arXiv:2310.19782; A. Y. Joe, et al., Phys. Rev. Lett. 132, 056303 (2024)]. A
two-dimensional (2D) metal-insulator transition (MIT) is demonstrated in the
Columbia sample at low densities, a regime where the formation of a Wigner
crystal (WC) is theoretically anticipated in the absence of disorder. We employ
the finite-temperature Boltzmann theory to understand the low-temperature
transport properties of monolayer TMDs, taking into account realistic disorder
scattering. We analyze the experimental results, focusing on the 2D MIT
behavior and the influence of temperature and density on mobility and
resistivity in the metallic phase. We provide a discussion of the nontrivial
carrier density dependence of our transport results. Our analysis elucidates
the linear-in-$T$ resistivity in the metallic phase, attributing it to Friedel
oscillations associated with screened charged impurities. Furthermore, we
explore whether Coulomb disorder could lead to the MIT through either a quantum
Anderson localization transition or a classical percolation transition. Our
theoretical estimates of the disorder-induced MIT critical densities, although
smaller, are within a factor of ~2 of the experimental critical density. We
examine the exceptionally high melting temperature ~10 K of WCs observed
experimentally in the MoSe$_2$ systems at low density, an order of magnitude
larger than the pristine melting temperature. This suggests that the observed
2D low-density MIT behavior is likely a result of the complex interplay between
disorder effects and interaction-driven WC physics, offering a comprehensive
understanding of the low-temperature transport phenomena in TMD monolayers.
