Web API in WebForms app returning XML instead of JSON

I added Web API to my existing Web Forms (NOT MVC) ASP.NET application using these instructions:

http://blogs.msdn.com/b/henrikn/archive/2012/02/23/using-asp-net-web-api-with-asp-net-web-forms.aspx

http://dotnet.dzone.com/articles/web-api-aspnet-web-forms

But, they return XML.  

<ArrayOfstring xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
   <string>value1</string>
   <string>value2</string>
</ArrayOfstring>

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I tried using JSON.NET...

    public string Get(int id)
    {
        DataTable person = MemberFunctions.GetContactInfo(id);
        return JsonConvert.SerializeObject(person);   
    }

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and it generates the JSON I need but it is still wrapped in XML tags:

<string xmlns="http://schemas.microsoft.com/2003/10/Serialization/">
    [{"Prefix":"Dr","FirstName":"Josh",   ....etc.......   }]
</string>

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How can I get WebAPI in a WebForms app to return valid JSON instead of XML, preferably using JSON.NET?