This constant is the conductance quantum (`G_0`) specifying the quantized electrical conductance unit.
The quantum conductance is defined as `G_0` = `2e^2/h` = 7.7480917346(25)×`10^(−5)` S.
This constant, the conductance quantum, `G_0`, is specified with a standard uncertainty (standard deviation) of 0.0000000025 x `10^(-5)` S
See Uncertainty of Measurement Results, a discussion provided by NIST of the application of uncertainty to the documented constants.
This value is derived from the measurement of the conductance of a quantum point contact
The basis for the conductance quantum derives from the Heisenberg Uncertainty Principle: the minimum energy-time uncertainty is ΔEΔt ~ h, where h is the Planck Constant (also in J*S). The current, I, in a quantum channel is e/T (T is transit time of the current represented by the electron charge, e.
When there is a potential, voltage V, an energy `E = e*V` results. The energy uncertainty is approximately E and the time uncertainty is approximately T. This gives us ΔEΔt ~ (eV)(e/I) ~ h. From G = I/V which describes the electrical conductance, we have G ~ `e^2/h`.