  We show that a class of weak three-dimensional topological insulators feature
one-dimensional Dirac electrons on their surfaces. Their hallmark is a
line-like energy dispersion along certain directions of the surface Brillouin
zone. These one-dimensional Dirac line degeneracies are topologically protected
by a symmetry that we refer to as in-plane time-reversal invariance. We show
how this invariance leads to Dirac lines in the surface spectrum of stacked
Kane-Mele systems and more general models for weak three-dimensional
topological insulators.
