Free-electron model for mesoscopic force fluctuations in nanowires

When two metal electrodes are separated, a nanometer-sized wire (nanowire) is formed just before the contact breaks. The electrical conduction measured during this retraction process shows signs of quantized conductance in units of $G_0{=2e}^2/h$. Recent experiments show that the force acting on the wire during separation fluctuates, which has been interpreted as being due to atomic rearrangements. In this paper we use a simple free-electron model, for two simple geometries, and show that the electronic contribution to the force fluctuations is comparable to the experimentally found values, about 2 nN.