  We have fabricated a series of ex situ copper sheathed powder-in-tube MgB2
wires with 20% by volume Ag, Pb, In, and Ga metal added to the MgB2 powder. We
find the transport critical current of these wires increases significantly with
the addition of specific metals to the core filament. In particular, the
critical current density (Jc) of the MgB2/Ga(20%) wire is in excess of 5x10^4
A/cm^2 at 10K in self field, nearly 50 times that of the MgB2/Ag(20%) wire. The
temperature dependent Jc of all wires is well described as an ensemble of clean
S/N/S junctions in which the relevant parameters are the average thickness of
the N layer, the critical temperature of the S layer, and a scaling term
related to Jc at zero temperature. Eliminating the differences in the filament
microstructure as the primary cause of the enhanced Jc, we suggest that Jc is
determined by the magnitude of the proximity effect induced superconductivity
in the normal metal layer, which is known to be proportional to the
electron-electron interaction in N. We present one-dimensional material
specific calculations that support this, and zero-field cooled DC magnetic
susceptibility data that confirm an increased number of well-connected
superconducting grains exist in the composite wires that contain metal
additions with large electron-electron interactions and long electron mean free
paths.
