  Material line defects are one-dimensional structures but the search and proof
of electron behaviour consistent with the reduced dimension of such defects has
been so far unsuccessful. Here we show using angle resolved photoemission
spectroscopy that twin-grain boundaries in the layered semiconductor MoSe2
exhibit parabolic metallic bands. The one-dimensional nature is evident from a
charge density wave transition, whose periodicity is given by kF/{\pi},
consistent with scanning tunneling microscopy and angle resolved photoemission
measurements. Most importantly, we provide evidence for spin- and
charge-separation, the hallmark of one-dimensional quantum liquids. Our studies
show that the spectral line splits into distinctive spinon and holon
excitations whose dispersions exactly follow the energy-momentum dependence
calculated by a Hubbard model with suitable finite-range interactions. Our
results also imply that quantum wires and junctions can be isolated in line
defects of other transition metal dichalcogenides, which may enable quantum
transport measurements and devices.
