  We study chaotic properties of eigenstates for periodic quasi-1D waveguides
with "regular" and "random" surfaces. Main attention is paid to the role of the
so-called "gradient scattering" which is due to large gradients in the
scattering walls. We demonstrate numerically and explain theoretically that the
gradient scattering can be quite strong even if the amplitude of scattering
profiles is very small in comparison with the width of waveguides.
