Ballistic pomt contacts, defined in the two-dimensional electron gas of a GaAs-AlGaAs heterostructure, have been studied in zero magnetic field The conductance changes in quantized Steps of e 2 /nh when the width, controlled by a gate on top of the heterojunction, is vaned Up to sixteen Steps are observed when the pomt contact is widened from 0 to 360 nm An explanation is proposed, which assumes quantized transverse momentum in the pomt-contact region As a result of the high mobihty attamable in the twodimensional electron gas (2DEG) in GaAs-AlGaAs heterostructures it is now becoming feasible to study ballistic transport in small devices '" 6 In metals ideal tools for such studies are constnctions havng a width W and length L much smaller than the mean free path l e These are known äs Sharvin pomt contacts 7 Because of the ballistic transport through these constnctions, the resistance is determmed by the pomt-contact geometry only Point contacts have been used extensively for the study of elastic and melastic electron scattermg With use of biased pomt contacts, electrons can be mjected mto metals at energies above the Fermi level This allows the study of the energy dependence of the scattermg mechamsms 8 With the use of a geometry containmg two pomt contacts, with Separation smaller than l e , electrons mjected by a pomt contact can be focused mto the other contact, by the application of a magnetic field This technique (transverse electron focusmg) has been applied to the detailed study of Fermi surfaces 9 In this Letter we report the first expenmental study of the resistance of ballistic pomt contacts m the 2DEG of high-mobihty GaAs-AlGaAs heterostructures The smgle-pomt contacts discussed m this paper are part of a double-pomt-contact device The results of transverse electron focusmg m these devices will be published elsewhere '° The pomt contacts are dehned by electrostatic depletion of the 2DEG underneath a gate This method, which has been used by several authors for the study of l D conduction,' 1 offers the possibility to control the width of the pomt contact by the gate voltage Control of the width is not feasible in metal pomt contacts The classical expression for the conductance of a pomt contact m two dimensions (see below) is G=(e 2 /nh)k Y W/n(1) in which kf is the Fermi wave vector and W is the width of the contact This expression is vahd if l e » W and the Fermi wavelength λρ<ίί W The first condition is satisfied in our devices, which have a maximum width W mm «= 250 nm and l e =8 5 μηι The second condition should also hold when the devices have the maximum width We expect quantum effects to become important when the width becomes comparable to λρ, which is 42 nm m our devices In this way we are able to study the transition from classical to quantum ballistic transport through the pomt contactThe pomt contacts are made on high-mobility molecular-beam-epitaxy-grown GaAs-AlGaAs heterostructures The electron density of the matenal is 3 56xl0 15 /m 2 and the mobihty 85 m 2 /V s (at 0 6 K) These values ...

In this, the first in a series of three papers concerning the SuperCOSMOS Sky Survey (SSS), we give an introduction and user guide to the survey programme. We briefly describe other wide‐field surveys and compare them with our own. We give examples of the data, and make a comparison of the accuracies of the various image parameters available with those from the other surveys providing similar data; we show that the SSS data base and interface offer advantages over these surveys. Some science applications of the data are also described and some limitations discussed. The series of three papers constitutes a comprehensive description and user guide for the SSS.

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