  Electrical currents in a quantum spin Hall insulator are confined to the
boundary of the system. The charge carriers can be described as massless
relativistic particles, whose spin and momentum are coupled to each other.
While the helical character of those states is by now well established
experimentally, it is a fundamental open question how those edge states
interact with each other when brought in spatial proximity. We employ a
topological quantum point contact to guide edge channels from opposite sides
into a quasi-one-dimensional constriction, based on inverted HgTe quantum
wells. Apart from the expected quantization in integer steps of $2 e^2/h$, we
find a surprising additional plateau at $e^2/h$. We explain our observation by
combining band structure calculations and repulsive electron-electron
interaction effects captured within the Tomonaga-Luttinger liquid model. The
present results may have direct implications for the study of one-dimensional
helical electron quantum optics, Majorana- and potentially para-fermions.
