  We study ensembles of fermionic cold-atom quantum wires with tunable
transverse mode population and single-wire resolution. From in situ density
profiles, we determine the temperature of the atomic wires in the weakly
interacting limit and reconstruct the underlying potential landscape. By
varying atom number and temperature, we control the occupation of the
transverse modes and study the 1D-3D crossover. In the 1D limit, we observe an
increase of the reduced temperature $T/T_{F}$ at nearly constant entropy per
particle $S/N k_{B}$. The ability to probe individual atomic wires in situ
paves the way to quantitatively study equilibrium and transport properties of
strongly interacting 1D Fermi gases.
