The magnetoconductance of nominally identical, 2-μm-long, Au lines has been measured. A 0.7-μm-diam loop was attached to the end of one of the lines outside the probes in such a way that classically no current would flow through it. As expected, the line by itself showed only aperiodic conductance fluctuations. In contrast, the line with the attached loop also exhibited h/e periodic oscillations. The periodic oscillations are due to the quantum interference of electrons diffusing around the loop. These results clearly demonstrate that the voltage measured between contact probes separated by a distance on the order of an electron phase coherence length is strongly influenced by regions outside the contact probes.