Fe/Au(001) multilayers have been grown by molecular beam epitaxy using a new technique for the preparation of Au buffer layers on MgO(001) substrates. Scanning tunnelling microscopy and grazing incidence x-ray scattering reveal that the Au buffers have extremely low interface roughness and a long in-plane correlation length. X-ray scattering and diffraction show that multilayers grown on the new buffers, although having little difference in their crystalline structure, have much smoother interfaces than those grown on previous buffers. Compared with earlier multilayers with rougher interfaces, the giant magnetoresistance has increased by 20% due to more complete antiferromagnetic exchange coupling. Measurement of the saturation conductivity shows that the number of specular reflections occurring during electron channelling remains unchanged even though the interfacial roughness is reduced by a factor of six. It is postulated that the cause of this is either that spin-dependent scattering at the interface is being driven solely by the difference in the band structure, not by diffuse scatter at the interfaces, or that the wavelength of the transport electrons is such that the change in the roughness has no effect upon the specular scattering.