The quantum-limit Hall effect at $\nu =nh/eB\sim O\left(1\right)$ that hosts a variety of exotic quantum phenomena requires demanding strong magnetic field $B$ and low carrier density $n$. We propose to realize the quantum-limit Hall effect even in the presence of large carrier density residues $n_e$ and $n_h$ in a relatively weak and variable magnetic field $B$ in topological semimetals, where a single Fermi-surface contour allows both electron-type and hole-type carriers and approaches charge neutrality when $n_e\approx n_h$. The underlying filling factor $\nu =|n_e-n_h|h/eB$ also offers an example that violates Onsager's relation for quantum oscillations.

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• Geometric & topological phases
• Landau levels
• Quantum Hall effect
• Topological materials
• Weyl semimetal