  The interplay between electronic interactions and disorder is neglected in
the conventional Boltzmann theory of transport, yet can play an essential role
in determining the resistivity of unconventional metals. When quasiparticles
are long-lived, one can account for these intertwined effects by solving
spatially inhomogeneous Boltzmann equations. Assuming smooth disorder and
neglecting umklapp scattering, we solve these inhomogeneous kinetic equations
and compute the electrical resistivity across the ballistic-to-hydrodynamic
transition. An important consequence of electron-electron interactions is the
modification of the momentum relaxation time; this effect is ignored in the
conventional theory. We characterize precisely when interactions enhance the
momentum scattering rate, and when they decrease it. Our approach unifies
existing semiclassical theories of transport and reveals novel transport
mechanisms. In particular, we explain how the resistivity can be proportional
to the rate of momentum-conserving collisions. We compare this result with
existing transport mysteries, including the disorder-independent $T^2$
resistivity of many Fermi liquids, and the linear-in-$T$ "Planckian-limited"
resistivity of many strange metals.
