The magnetoresistance of small wires and loops is described. At low temperatures, the electrons retain quantum mechanical phase coherence for distances of a micron or so, which is the size scale over which the measurements are made. The random interference of the electron wavefunctions appears as random fluctuations in the magnetoresistance, and in loops, the Aharonov–Bohm effect causes periodic oscillations. The resistance varies randomly with the Fermi energy of the electrons, with the voltage across the sample, and with any transverse voltages applied by external gates. The nonlocal character of the wavefunction leads to resistance fluctuations in configurations where classical physics predicts null results. Throughout the discussion the physics is explained for those new to the field.