  Electronic transport in the regime where carrier-carrier collisions are the
dominant scattering mechanism has taken on new relevance with the advent of
ultraclean two-dimensional materials. Here we present a combined theoretical
and experimental study of ambipolar hydrodynamic transport in bilayer graphene
demonstrating that the conductivity is given by the sum of two Drude-like terms
that describe relative motion between electrons and holes, and the collective
motion of the electron-hole plasma. As predicted, the measured conductivity of
gapless, charge-neutral bilayer graphene is sample- and temperature-independent
over a wide range. Away from neutrality, the electron-hole conductivity
collapses to a single curve, and a set of just four fitting parameters provides
quantitative agreement between theory and experiment at all densities,
temperatures, and gaps measured. This work validates recent theories for
dissipation-enabled hydrodynamic conductivity and creates a link between
semiconductor physics and the emerging field of viscous electronics.
