  Recent experiments on twisted bilayer graphene (tBG) close to magic angle
show that a small relative rotation in a van der Waals heterostructure greatly
alters its electronic properties. We consider various scattering mechanisms and
show that the carrier transport in tBG is dominated by a combination of charged
impurities and acoustic gauge phonons. Charged impurities still dominate at low
temperature and densities because of the inability of Dirac fermions to screen
long-range Coulomb potentials at charge neutrality; however, the gauge phonons
dominate for most of the experimental regime because although they couple to
current, they do not induce charge and are therefore unscreened by the large
density of states close to magic angle. We show that the resistivity has a
strong monotonically decreasing carrier density dependence at low temperature
due to charged impurity scattering, and weak density dependence at high
temperature due to gauge phonons. Away from charge neutrality, the resistivity
increases with temperature, while it does the opposite close to the Dirac
point. A non-monotonic temperature dependence observed only at low temperature
and carrier density is a signature of our theory that can be tested in
experimentally available samples.
