correct magnetic field units

Magnetic fields are now expressed in units of magnetic flux quantum
per unit area.

kwant/physics/gauge.py

@@ -787,7 +787,7 @@ def calculate_phases(loops, pos, previous_phase, flux):
     for loop in loops:
         tail, head = loop[-1], loop[0]
         integral = flux([pos(p) for p in loop])
-        phase = np.exp(2j * np.pi * integral)
+        phase = np.exp(1j * np.pi * integral)
         phases[tail, head] = phase / previous_phase(phases, loop)
     return phases
 
@@ -1001,6 +1001,9 @@ class magnetic_gauge:
             If callable, takes a position and returns the
             magnetic field at that position. Can be a scalar if
             the system is 1D or 2D, otherwise must be a vector.
+            Magnetic field is expressed in units :math:`φ₀ / l²`,
+            where :math:`φ₀` is the magnetic flux quantum and
+            :math:`l` is the unit of length.
         *lead_fields : scalar, vector or callable
             The magnetic fields to apply to each of the leads, in
             the same format as 'syst_field'. In addition, if a callable

kwant/physics/tests/test_gauge.py

@@ -211,7 +211,7 @@ def _test_phase_loops(syst, phases, loops):
     for loop_kind, loop_flux in loops:
         for loop in available_loops(syst, loop_kind):
             loop_phase = np.prod([phases(a, b) for a, b in loop_to_links(loop)])
-            expected_loop_phase = np.exp(2j * np.pi * loop_flux)
+            expected_loop_phase = np.exp(1j * np.pi * loop_flux)
             assert np.isclose(loop_phase, expected_loop_phase)