  Accurate determination of carrier transport properties in two-dimensional
(2D) materials is critical for designing high-performance nano-electronic
devices and quantum information platforms. While first-principles calculations
effectively determine the atomistic potentials associated with defects and
impurities, they are ineffective for direct modeling of carrier transport
properties at length scales relevant for device applications. Here, we develop
a scalable first-principles-informed quantum transport theory to investigate
the carrier transport properties of 2D materials. We derive a non-asymptotic
quantum scattering framework to obtain transport properties in proximity to
scattering centers. We then bridge our scattering framework with
$\textit{k}\cdot\textit{p}$ perturbation theory, with inputs from
first-principles electronic structure calculations, to construct a versatile
multiscale formalism that enables modeling of realistic devices at the
mesoscale. Our formalism also accounts for the crucial contributions of
decaying evanescent modes across heterointerfaces. We apply this formalism to
study electron transport in lateral transition-metal dichalcogenide (TMDC)
heterostructures and show that material inclusions can lead to an enhancement
in electron mobility by an order of magnitude larger than pristine TMDCs.
