For sub-10 nm thin metallic films, very little knowledge is available so far on how electron scattering at surface and grain boundaries reduces the thermal transport. This work reports on the first time characterization of the thermal and electrical conductivities of gold films of 6.4 nm average thickness. The electrical (σ) and thermal (k) conductivities of the Au film are found to be reduced dramatically from their bulk counterparts by 93.7% (σ) and 80.5% (k). Its Lorenz number is measured as 7.44 × 10(-8) W Ω K(-2), almost a twofold increase from the bulk value. The Mayadas-Shatzkes model is used to interpret the experimental results and reveals very strong electron reflection (77%) at grain boundaries.