DlFFRACTiON of light by an aperture is a well-known manifestation of the \vave nature of light. The most familiär case is that of an incident plane wave, which is diffracted into a spatial pattern that is sensitive to the properties of the aperture: the ratio of transmitted power to incident flux (the transmission cross-section σ) depends in a complicated way on the aperture area A (refs 1-3). For diffuse (that is, Isotropie rather than plane-wave) Illumination, however, the Situation is much simpler 4 : in three dimensions, σ increases with A in a series of Steps of equal height Α 2 /2ττ (where A is the wavelength of the light), and is thus independent of the detailed aperture shape. A similar simplification occurs for two-dimensional diffuse Illumination of a slit: the transmission cross-section per unit slit length increases in stepwise fashion äs a function of the slit width W, with Steps of height A/2 occurring whenever W = ηλ/2 (π = l, 2,3,...)-that is, whenever a new mode is enabled in the slit. Although the optical transmission characteristics of slits have been studied extensively for plane-wave Illumination 5 " 8 , we know of no investigation of this predicted staircase dependence for diffuse Illumination. Here we report the observation of such an effect, and suggest that it may play a part in any process of wave propagation through a constriction.The argument of ref. 4 is based on the analogy with the recently discovered quantization of conductance in ballistic electron transport 9 ' 10 . The conductance of a point contact in a two-dimensional electron gas increases in steps of 2e 2 / h äs its width is increased (e is the Charge on an electron). The origin of this effect is the quantization of the electrons' transverse momentum owing to lateral confinement within the point contact. This leads to the formation of one-dimensional sub-bands in the conduction band (analogous to the formation of transverse λ = 1.55 μιη

Ampoule diffusion of Zn in InP gives rise to donor-acceptor photoluminescence transitions with peak positions dependent on the rate of cooling after diffusion. Subsequent annealing in an atmosphere without Zn causes a change in peak position. Luminescence peaks between 1.30 and 1.38 eV are found. These peaks are described as being due to transitions between various Zn interstitial donor levels and the Zn substitutional acceptor level. The luminescence results are correlated with results of secondary ion mass spectrometry and Schottky barrier capacitance-voltage measurements, and are consistent with an earlier medel in which Zn diffuses as an interstitial donor and is incorporated both as an interstitial donor and as a substitutional acceptor. 5549

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