  We develop a method to extract the universal conductance of junctions of
multiple quantum wires, a property of systems connected to reservoirs, from
static ground-state computations in closed finite systems. The method is based
on a key relationship, derived within the framework of boundary conformal field
theory, between the conductance tensor and certain ground state correlation
functions. Our results provide a systematic way of studying quantum transport
in the presence of strong electron-electron interactions using efficient
numerical techniques such as the standard time-independent density-matrix
renormalization-group method. We give a step-by-step recipe for applying the
method and present several tests and benchmarks. As an application of the
method, we calculate the conductance of the M fixed point of a Y junction of
Luttinger liquids for several values of the Luttinger parameter $g$ and
conjecture its functional dependence on $g$.
