Quantum Transport in Fully Characterized Atomic-Size Contacts Alexei Marchenkov School of Physics, Georgia Tech   Significant quantum effects in transport properties appear when at least one dimension of a ballistic electronic conductor becomes shorter than the so-called coherence length. In metals, this regime is achieved in constrictions containing just a few atoms, where current is transported through a small number of conductance channels. My focus will be on atomic-size contacts made of superconducting metals. Due to the interplay between the macroscopic phase coherence of the order parameter and size effects, the physics of these systems is extremely rich. In particular, a new conductance mechanism, called multiple Andreev reflections (MAR), dominates transport at low voltages. Analysis of highly non-linear current-voltage curves yields the number of conductance channels and their individual transport characteristics. I will also discuss the significant influence of the electromagnetic environment on the properties of small-scale superconducting devices and circuits.