Some remarks on 't Hooft's S-matrix for black holes
  We discuss the limitations of 't Hooft's proposal for the black hole S-matrix. We find that the validity of the S-matrix implies violation of the semi-classical approximation at scales large compared to the Planck scale. We also show that the effect of the centrifugal barrier on the S-matrix is crucial even for large transverse distances. 
N.Itzhaki