  We report on electric-field and temperature dependent transport measurements
in exfoliated thin crystals of Bi$_{2}$Se$_{3}$ topological insulator. At low
temperatures ($< 50$ K) and when the chemical potential lies inside the bulk
gap, the crystal resistivity is strongly temperature dependent, reflecting
inelastic scattering due to the thermal activation of optical phonons. A linear
increase of the current with voltage is obtained up to a threshold value at
which current saturation takes place. We show that the activated behavior, the
voltage threshold and the saturation current can all be quantitatively
explained by considering a single optical phonon mode with energy $\hbar \Omega
\approx 8$ meV. This phonon mode strongly interacts with the surface states of
the material and represents the dominant source of scattering at the surface at
high electric fields.
