Schematic representation of a persistent current in a mesoscopic metal ring threaded by a magnetic flux quantum, . For rings in size and at a temperature , the flux quantum induces a persistent current due to the Aharonov-Bohm effect. Elastic scattering from static impurities (light blue spheres) does not destroy the phase coherence of the wave functions and persistent currents can flow even though the metal has a finite resistance. The phase coherence of the electron waves is more sensitive to spin-flip scattering from magnetic impurities (red arrows). However, while a concentration of 1 ppm magnetic impurities would be enough to destroy superconductivity in these metal rings, it is not sufficient to quench persistent currents in the normal state.