  We study the resistivity scaling in nanometer-sized metallic wires due to
surface roughness and grain-boundaries, currently the main cause of electron
scattering in nanoscaled interconnects. The resistivity has been obtained with
the Boltzmann transport equation, adopting the relaxation time approximation
(RTA) of the distribution function and the effective mass approximation for the
conducting electrons. The relaxation times are calculated exactly, using
Fermi's golden rule, resulting in a correct relaxation time for every sub-band
state contributing to the transport. In general, the relaxation time strongly
depends on the sub-band state, something that remained unclear with the methods
of previous work. The resistivity scaling is obtained for different roughness
and grain-boundary properties, showing large differences in scaling behavior
and relaxation times. Our model clearly indicates that the resistivity is
dominated by grain-boundary scattering, easily surpassing the surface roughness
contribution by a factor of 10.
