  Transport in topological matter has shown a variety of novel phenomena over
the last decade. Although numerous transport studies have been conducted on
three-dimensional topological insulators (3D-TIs), study of ballistic motion
and thus exploration of potential landscapes on a hundred nanometer scale is
for the prevalent TI materials almost impossible due to their low carrier
mobility. Therefore it is unknown whether helical Dirac electrons in TIs, bound
to interfaces between topologically distinct materials, can be manipulated on
the nanometer scale by local gates or locally etched regions. Here we impose a
submicron periodic potential onto a single surface of Dirac electrons in high
mobility strained mercury telluride (HgTe), which is a strong TI. Pronounced
geometric resistance resonances constitute the first observation of a ballistic
effect in 3D-TIs.
