  We report on a study of an ultrathin topological insulator film with
hybridization between the top and bottom surfaces, placed in a quantizing
perpendicular magnetic field. We calculate the full Landau level spectrum of
the film as a function of the applied magnetic field and the magnitude of the
hybridization matrix element, taking into account both the orbital and the
Zeeman spin splitting effects of the field. For an undoped film, we find a
quantum phase transition between a state with a zero Hall conductivity and a
state with a quantized Hall conductivity equal to $e^2/h$, as a function of the
magnitude of the applied field. The transition is driven by the competition
between the Zeeman and the hybridization energies.
