DOUBLING OF SCATTERING PHASE-SHIFTS FOR 2-DIMENSIONAL STRINGS

We discuss the origin of the leg factors appearing in 2D string theory . Computing in the world sheet framework we use the semiclassical meth od to study string amplitudes at high energy. We show that in the case of a simplest two-point amplitude these factors correspond entirely t o the time delay for reflection off the Liouville wall. Our semiclassi cal calculation reveals that the string longitudinal modes, although n onpropagating in 2D spacetime, have the effect of doubling the phase s hift. Particular emphasis is put on comparison with the point particle (center of mass) case. A general method is then given for calculating an arbitrary amplitude semiclassically.