  Low-temperature electronic systems require electrical leads which have low
electrical resistance to provide bias current I without excessive voltage drop
V. But proper cryogenic design also requires high thermal resistance to
maintain a minimum heat leak Q from the hot temperature T[hot] to the cold
temperature T[cold]. By the Wiedemann-Franz law, these requirements are in
direct conflict, and the optimal configuration takes a particularly simple
universal approximate form for the common case that T[cold] << T[hot]: Q/I = V
= 3.6 kT[hot]/e. This is applied here to the cryopackaging of RSFQ
superconducting circuits on a 4K cryocooler, but is equally applicable to other
cryogenic systems such as a superconducting sensor array at low and ultra-low
temperatures.
