Four-probe STM is a “nano” version of a conventional four-probe station; it combines STM local imaging and spectroscopy functions with four-point contact electrical transport capability in a well-controlled sample environment at temperatures down to 8 K.
 In this talk, I will give a brief overview on the capabilities of ORNL four-probe STM, and then provide a couple of examples to demonstrate how we are using this unique platform to study the distribution and flow of electrons over multiple length scales, from individual atoms, molecules, to nanostructures and mesoscopic systems. My focus will be on the measurements of individual grain boundary resistance in copper interconnect nanowires and the one-dimensional metallic conductance stabilized by interwire coupling in ultra narrow silicide nanowires grown on a silicon surface. The goal of this research is to establish the relationship between transport functionalities and local structural and electronic properties down to atomic scale. This research was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.