Josephson junctions using Dayem bridges made from unusually thin films have been operated as mixers. The thickness of the film was made small to increase the r.f. impedance of the device and so to improve r.f. matching, but such thin films often fail to act as Josephson junctions because of surface roughness. In the device reported here, attempts were made to obtain specular reflection of the electron wave functions from the surface. Our devices behaved as good Josephson junctions and gave an internal mixing efficiency of 4.5% at 32 GHz.