Quantum Coherent Transport of 1D Ballistic States in Second-Order Topological Insulator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>Bi</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</

We investigate quantum transport in micrometer-sized single crystals of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mrow> <a:mi>Bi</a:mi> </a:mrow> <a:mrow> <a:mn>4</a:mn> </a:mrow> </a:msub> </a:mrow> <a:mrow> <a:msub> <a:mrow> <a:mi>Br</a:mi> </a:mrow> <a:mrow> <a:mn>4</a:mn> </a:mrow> </a:msub> </a:mrow> </a:math> , a material predicted to be a second-order topological insulator. 1D topological states with long phase coherence times are revealed via the modulation of quantum interference with magnetic field and gate voltage. In particular, we demonstrate the existence of Aharonov-Bohm interference between 1D ballistic states several micrometers long, that we identify as phase-coherent hinge modes on neighboring step edges at the crystal surface. These Aharonov-Bohm oscillations are made possible by a disordered phase-coherent contact region, the existence of which is confirmed by scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy (STEM-EDX) of FIB lamellae. Their coherent nature modulates the transmission of the 1D edge states, leading to weak antilocalization and universal conductance fluctuations with surprisingly large characteristic fields and a strongly anisotropic behavior. These complementary experimental results provide a comprehensive, coherent description of quantum transport in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:msub> <c:mrow> <c:mi>Bi</c:mi> </c:mrow> <c:mrow> <c:mn>4</c:mn> </c:mrow> </c:msub> </c:mrow> <c:mrow> <c:msub> <c:mrow> <c:mi>Br</c:mi> </c:mrow> <c:mrow> <c:mn>4</c:mn> </c:mrow> </c:msub> </c:mrow> </c:math> and establish the material as a second-order topological insulator with topologically protected 1D ballistic states.