I expect a client cert will return after starting System.Net.HttpListener in "Negotiate" mode

I want to write a HttpListener requires browser to submit its client cert before response.  When start following code, the browser prompts for password only and nothing is returned in the server side statement .GetClientCertificate() .  Instead, the response headers("Authorization") has a long base64 string [Negotiate TlRMTVNTUAADA...].  I suspect it is the client cert that I want.  However, I fail to create the cert from it.

What is wrong

Imports System.Security.Cryptography.X509Certificates
Module modClientCert
    Private xListener As New System.Net.HttpListener
    Public Sub Main()
        xListener.Prefixes.Add("http://192.168.2.103/")
        xListener.AuthenticationSchemes = AuthenticationSchemes.Negotiate
        xListener.Start()

        Dim mContext As HttpListenerContext = xListener.GetContext
        With mContext.Request
            Dim mIsAuth As Boolean = .IsAuthenticated ' Return True
            Dim mCert As X509Certificate2 = .GetClientCertificate() ' Return nothing
            Dim mCertErr As Integer = .ClientCertificateError ' Return 0
            Dim mNegotiate As string = .Headers("Authorization") ' Return Negotiate TlRMTVNTUAADAAAAGAAYAI4AAAAYABgApgAAABoAGgBIAAAAGAAYAGIAAAAUABQAegAAAAAAAAC+AAAABYKIogUBKAoAAAAPMQA5ADIALgAxADYAOAAuADIALgAxADAAMwBhAGQAbQBpAG4AcwB0AHIAYQB0AG8AcgBKAE8ASABOAFMATwBOAFQANAAzAGD3k7ByYKqeAAAAAAAAAAAAAAAAAAAAAIK+w+qKJqwaXCPR6Lxtmei3e7Pwy337rQ==
            Dim mCert_Raw() As Byte = System.Convert.FromBase64String(Mid(mNegotiate, 11))
            Dim mCert_x As New X509Certificate2(mCert_Raw) ' Throw System.Security.Cryptography.CryptographicException : Cannot find the requested object.
        End With
   
    End Sub
End Module