Universal Minimum Heat Leak on Low-Temperature Metallic Electrical Leads
  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. 
Alan M. Kadin