Different Fractional Charges from Auto- and Cross-Correlation Noise in Quantum Hall States without Upstream Modes

Fractional charges of anyons can be extracted from shot noise in two ways. One can use either the autocorrelation noise of the current in one drain or the cross-correlation noise between two drains on the two sides of the device. The former approach typically overestimates the charge. This may happen due to upstream edge modes. We propose a mechanism for the excess autocorrelation noise without upstream modes. It applies to systems with multiple copropagating edge modes and assumes that the noise is measured at a low but nonzero frequency.