  The operation of many classical and quantum systems in nonequilibrium steady
state is constrained by cost-precision (dissipation-fluctuation) tradeoff
relations, delineated by the thermodynamic uncertainty relation (TUR). However,
coherent quantum electronic nanojunctions can escape such a constraint, showing
finite charge current and nonzero entropic cost with vanishing current
fluctuations. Here, we analyze the absence, and restoration, of cost-precision
tradeoff relations in fermionic nanojunctions under different affinities:
voltage and temperature biases. With analytic work and simulations, we show
that both charge and energy currents can display the absence of cost-precision
tradeoff if we engineer the transmission probability as a boxcar function --
with a perfect transmission and hard energy cutoffs. Specifically for charge
current under voltage bias, the standard TUR may be immediately violated as we
depart from equilibrium, and it is exponentially suppressed with increased
voltage. However, beyond idealized, hard-cutoff energy-filtered transmission
functions, we show that realistic models with soft cutoffs or imperfect
transmission functions follow cost-precision tradeoffs, and eventually recover
the standard TUR sufficiently far from equilibrium. The existence of
cost-precision tradeoff relations is thus suggested as a generic feature of
realistic nonequilibrium quantum transport junctions.
