Metallic nanofilms are of great importance in integrated circuit design and electronic devices. Understanding energy dissipation and transport in metallic nanofilms is essential to practical thermal management. The Wiedemann-Franz (WF) law states a precisely fixed ratio by which the electrons transport heat and charge, providing a basic rule to determine the thermal properties. Hitherto no bulk material has been known to violate the WF law. We report compelling evidence for the breakdown of the WF law in polycrystalline gold nanofilms at low temperatures, the Lorenz number increases notably with decreasing temperature. Our results show that the electrons dominate in heat transport at high temperatures, leading to a constant Lorenz number. While below 401K, inelastic electron scattering at grain boundaries becomes significant and part of the electron energy is transferred to phonons. Correspondingly, the phonon thermal conductivity is increased and the WF law is violated. A detailed kinetic theoretical model has been developed to investigate several phonon scattering mechanisms in depth and matches well with the experimental results. (C) 2013 Elsevier Ltd. All rights reserved.