  We discuss the quantum bound on chaos in the context of the free propagation
of a particle in an arbitrarily curved surface at low temperatures. The
semiclassical calculation of the Lyapunov exponent can be performed in much the
same way as the corresponding one for the `Loschmidt echo'.The bound appears
here as the impossibility to scatter a wave, by effect of the curvature, over
characteristic lengths smaller than the deBroglie wavelength.
