  We report gated terahertz Faraday angle measurements on epitaxial Bi2Se3 thin
films capped with In2Se3. A plateau is observed in the real part of the Faraday
angle at an onset gate voltage corresponding to no band bending at the surface
which persists into accumulation. The plateau is two orders of magnitude
flatter than the step size expected from a single Landau Level in the low
frequency limit, quantized in units of the fine structure constant. At 8 T, the
plateau extends over a range of gate voltage that spans an electron density
greater than 14 times the quantum flux density. Both the imaginary part of the
Faraday angle and transmission measurements indicate dissipative off-axis and
longitudinal conductivity channels associated with the plateau.
