  We measure the temperature-dependent carrier density and resistivity of the
topological surface state of thin exfoliated Bi2Se3 in the absence of bulk
conduction. When the gate-tuned chemical potential is near or below the Dirac
point the carrier density is strongly temperature dependent reflecting thermal
activation from the nearby bulk valence band, while above the Dirac point,
unipolar n-type surface conduction is observed with negligible thermal
activation of bulk carriers. In this regime linear resistivity vs. temperature
reflects intrinsic electron-acoustic phonon scattering. Quantitative comparison
with a theoretical transport calculation including both phonon and disorder
effects gives the ratio of deformation potential to Fermi velocity D/\hbarvF =
4.7 {\AA}-1. This strong phonon scattering in the Bi2Se3 surface state gives
intrinsic limits for the conductivity and charge carrier mobility at room
temperature of ~550 {\mu}S per surface and ~10,000 cm2/Vs.
