  Interlayer exciton diffusion is studied in atomically-reconstructed
MoSe2/WSe2 heterobilayers with suppressed disorder. Local atomic registry is
confirmed by characteristic optical absorption, circularly-polarized
photoluminescence, and g-factor measurements. Using transient microscopy we
observe propagation properties of interlayer excitons that are independent from
trapping at moir\'e- or disorder-induced local potentials. Confirmed by
characteristic temperature dependence for free particles, linear diffusion
coefficients of interlayer excitons at liquid helium temperature and low
excitation densities are almost 1000 times higher than in previous
observations. We further show that exciton-exciton repulsion and annihilation
contribute nearly equally to non-linear propagation by disentangling the two
processes in the experiment and simulations. Finally, we demonstrate effective
shrinking of the light-emission over time across several 100's of picoseconds
at the transition from exciton- to the plasma-dominated regimes. Supported by
microscopic calculations for bandgap renormalization to identify Mott
threshold, this indicates transient crossing between rapidly expanding,
short-lived electron-hole plasma and slower, long-lived exciton populations.
