  We investigate lateral p-n junctions, electrostatically defined in 14 nm-wide
HgTe-based quantum wells (QWs) with inverted band structure. The p-n junctions
resistances are close to $h/2e^2$, consistent with some previous experiments on
8-10 nm QWs, and the current-voltage characteristics are highly linear,
indicating the transport via ballistic helical edge states. Shot noise
measurements are performed in order to further verify the underlying transport
mechanism. We discuss the role of unknown inelastic relaxation rates in the
leads and in the edge channels for the correct interpretation of the noise
data. Although the interpretation in favor of the helical edge states seems
more consistent, a definite conclusion can not be drawn based on the present
experiment. Our approach looks promising for the study of short quasi-ballistic
edges in topological insulators (TIs) in suitable geometry.
