Mass Spectrum for Black Holes in Generic 2-D Dilaton Gravity
  Two arguments for the quantization of entropy for black holes in generic 2-D dilaton gravity are summarized. The first argument is based on reduced quantization of the only physical observables in the theory, namely the black hole mass and its conjugate momentum, the Killing time separation. The second one uses the exact physical mass eigenstates for Euclidean black holes found via Dirac quantization. Both methods give the same spectrum: the black hole entropy must be quantized $S= 2\pi n/G$. 
A. Barvinsky (Lebedev Physics Institute) and G. Kunstatter (University   of Winnipeg)