  It has been argued recently that string theory effects qualitatively modify
the effective black hole geometry experienced by modes with radial momentum of
order $1/\sqrt{\alpha'}$. At tree level, these $\alpha'$-effects can be
explicitly worked out in two-dimensional string theory, and have a natural
explanation in the T-dual description as coming from the integration of the
zero-mode of the linear dilaton, what yields a contribution that affects the
scattering phase-shift in a peculiar manner. It has also been argued that the
phase-shift modification has its origin in a region of the moduli space that
does not belong to the exterior black hole geometry, leading to the conclusion
that at high energy the physics of the problem is better described by the dual
model. Here, we elaborate on this argument. We consider the contribution of
worldsheet instantons in the 2D Euclidean black hole sigma-model and study its
influence on the phase-shift at high energy.
