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Summary form only given.Duality is often used to highlight the difference between quantum mechanics and classical physics. It expresses the fact, that observation of interference and which way information are mutually exclusive. It has been vividly discussed recently, whether or not the loss of interference in an arbitrary which-way experiment can always be attributed to mechanical forces. While a lot of Gedanken experiments have been proposed, there are only a few experimental results. We now report on an experiment that addresses precisely this question. Our atom interferometer is based on Bragg scattering from standing light waves.