1^st part

Acurrent is injected into the wire. The electrons start moving, collide with the wires' metallic ions causing them to migrate. Consequence: the wire becomes thinner in the very spot where it was already narrower in the beginning and the wire heats up more and more. 

2^nd part

The temperature is then lowered below the wire's critical temperature in order to study its superconductivity. In the superconducting state, the electrons form Cooper pairs (in green). In the middle, the fluctuations are such that the electrons break away from each other ("normal" electrons, blue): superconductivity is destroyed at this point.

Thermal and quantum depletion of superconductivity in narrow junctions created by controlled electromigration, Xavier D.A. Baumans et al. NATURE COMMUNICATIONS | 7:10560 | DOI: 10.1038/ncomms10560 |