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VOLUME 83, NUMBER 5 PHYSICAL REVIEW LETTERS 2A UGUST 1999Comment on “Fractal Conductance Fluctuations in Fig. 1(b)] which track the curves and reveal the samedeviation from linearity at largeDB.in a Soft-Wall Stadium and a Sinai Billiard”2Since the theory formulates FCF in terms ofDG [3],Sachrajda et al. [1] claim their investigations of semi- this is an obvious method for their detection. A box count-conductor billiards provide the first observation of fractal ing parameterN is also used to detect FCF in [1]. The frac-2dconductance fluctuations (FCF) over 2 orders of magni- tal dimension d is calculated using N DB , wheretude in magnetic field B. Here we reanalyze the data for 1, d, 2. Figure 1(d) shows log N vs log DB for10 10the Sinai billiard (SB) in [1] plus previously reported data the traces of Fig. 1(a). For the nonfractal model curve[2] for this billiard and show that the FCF are limited to (bottom), the gradient must be21 (corresponding todone order. 1) at smallDB, which is the gradient of the line shown. ForIf conductanceG vsB curves exhibit FCF, the variance DB. 0.3 mT, the periodicity in our model curve causes2of G increments scales with B increments as DG the associated log plot to deviate from linearity. Under-422DDB , where the fractal dimensionD obeys1, D, standing how the periodicity in the experimental GB2 [3]. The top three traces in Fig. 1(a) show experimen- curves affects the shapes of the log N vs log DB ...