A break junction is an electronic device which consists of two metal wires separated by a very thin gap, on the order of the inter-atomic spacing (less than a nanometer).
[1] As the wire breaks, the separation between the electrodes can be indirectly controlled by monitoring the electrical resistance of the junction.
After the gap is formed, its width can often be controlled by bending the substrate that the metal contacts lie on.
The conductance quantum has a value of 7.74×10−5 siemens, corresponding to a resistance increase of roughly 12.9 kΩ.
Break junctions are used to make electrical contacts to study single molecules.