  By combining analytical and numerical approaches, we theoretically
investigate the effect of fabrication imperfections, e.g. roughness at metal
interfaces, on nanometer metal-insulator-metal waveguides supporting slow
gap-plasmon modes. Realistic devices with vapor deposition- and
chemically-grown metal films are considered. We obtain quantitative predictions
for the attenuations induced by absorption and by backscattering, and
analytically derive how both attenuations scale with respect to the group
velocity. Depending on the material parameters and fabrication quality,
roughness-induced backscattering is find to be a significant additional source
of attenuation for small group velocities, which is often neglected in the
literature.
