  Investigating Protostellar Accretion (IPA) is a JWST Cycle~1 GO program that
uses NIRSpec IFU and MIRI MRS to obtain 2.9--28~$\mu$m spectral cubes of young,
deeply embedded protostars with luminosities of 0.2 to 10,000~L$_{\odot}$ and
central masses of 0.15 to 12~M$_{\odot}$. In this Letter, we report the
discovery of a highly collimated atomic jet from the Class~0 protostar
IRAS~16253$-$2429, the lowest luminosity source ($L_\mathrm{bol}$ = 0.2
$L_\odot$) in the IPA program. The collimated jet is detected in multiple
[Fe~II] lines, [Ne~II], [Ni~II], and H~I lines, but not in molecular emission.
The atomic jet has a velocity of about 169~$\pm$~15~km\,s$^{-1}$, after
correcting for inclination. The width of the jet increases with distance from
the central protostar from 23 to~60 au, corresponding to an opening angle of
2.6~$\pm$~0.5\arcdeg. By comparing the measured flux ratios of various fine
structure lines to those predicted by simple shock models, we derive a shock
{speed} of 54~km\,s$^{-1}$ and a preshock density of
2.0$\times10^{3}$~cm$^{-3}$ at the base of the jet. {From these quantities and
using a suite of jet models and extinction laws we compute a mass loss rate
between $0.4 -1.1\times10^{-10}~M_{\odot}$~yr~$^{-1}$.} The low mass loss rate
is consistent with simultaneous measurements of low mass accretion rate
($2.4~\pm~0.8~\times~10^{-9}~M_{\odot}$~yr$^{-1}$) for IRAS~16253$-$2429 from
JWST observations (Watson et al. in prep), indicating that the protostar is in
a quiescent accretion phase. Our results demonstrate that very low-mass
protostars can drive highly collimated, atomic jets, even during the quiescent
phase.
