  Whilst it has long been known that disorder profoundly affects transport
properties, recent measurements on a series of solid solution 3d-transition
metal alloys reveal two orders of magnitude variations in the residual
resistivity. Using ab-initio methods, we demonstrate that, while the carrier
density of all alloys is as high as in normal metals, the electron
mean-free-path can vary from ~10 {\AA} (strong scattering limit) to ~10$^3$
{\AA} (weak scattering limit). Here, we delineate the underlying electron
scattering mechanisms responsible for this disparate behavior. While spin
dependent site-diagonal disorder is always dominant, for alloys containing only
Fe, Co, and Ni the majority spin channel experiences negligible disorder
scattering, thereby providing a short circuit, while for Cr/Mn containing
alloys both spin channels experience strong disorder scattering due to an
electron filling effect. Unexpectedly, other scattering mechanisms (e.g.
displacement scattering) are found to be relatively weak in most cases.
