Quantum coherence: Just what is superconductivity?

Whether or not a superconductor is truly superconducting depends on its size and even its shape. In a geometry intermediate between one and two dimensions, it seems a thin film does not reach a state of zero resistance except at zero temperature.

Superconductivity seems like a magic example of quantum mechanics on a macroscopic scale. At low temperatures, certain materials undergo a thermodynamic phase transition at which they apparently lose all resistance to the flow of electricity and tend to expel any magnetic fields inside them.